Section 3 - At the Telescope with a CCD Camera
One of the reasons that SBIG autoguiders are often better than human guiders is that, rather than just stabbing the hand controller to bump the guide star back to the reticule, it gives a precise correction that is the duration necessary to move the guide star right back to its intended position. It knows how much correction is necessary for a given guiding error through the Calibrate Track command. The Calibrate Track command, which is used prior to autoguiding, exercises the telescope's drive corrector in each of the four directions, measuring the displacement of a calibration star after each move. Knowing the displacement and the duration of each calibration move calibrates the drive's correction speed. Once that is known, the CCD tracker gives the drive corrector precise inputs to correct for any guiding error.
When self guiding is selected by invoking the Self Guiding command under the Track Menu, the computer prompts the user for the exposure time for the tracking and imaging CCDs. Once these are entered, the computer takes and displays an image with the tracking CCD, and the user selects a guide star using the mouse. Guide stars that are bright, but not saturating, and isolated from other stars are preferred. Once the star is selected, the computer starts guiding the telescope. When the telescope corrections settle down (usually once the backlash is all taken up in the declination drive) the user starts the exposure by striking the space bar. The computer then integrates for the prescribed time while guiding the telescope, and downloads the image for display.
A calibration star should be chosen that is relatively bright and isolated. The calibration software can get confused if another star of comparable brightness moves onto the tracking CCD during a move. The unit will self guide on much fainter stars. Tests at SBIG indicate that the probability of finding a usable guide star on the tracking CCD is about 95% at F/6.3, in regions of the sky away from the Milky Way. If a guide star is not found the telescope position should be adjusted, or the camera head rotated by a multiple of 90 degrees to find a guide star. We recommend that the user first try rotating the camera 180 degrees. Rotating the camera will require recalibration of the tracking function.
3.9.5. Auto Grab
The Auto Grab command allows you to take a series of images at a periodic interval and log the images to disk. This can be invaluable for monitoring purposes such as asteroid searches or stellar magnitude measurements. You can even take
3.9.6. Color Imaging
The field of CCD color imaging is relatively new but expanding rapidly. Since all SBIG cameras are equipped with monochromatic CCDs, discriminating only light intensity, not color, some provision must be made in order to acquire color images. SBIG offers a color filter wheel, the
The color filter wheel allows remotely placing interference filters in front of the CCD in order to take multiple images in different color bands. These narrow band images are then combined to form a color image. With the SBIG system, a Red, Green and Blue filter are used to acquire three images of the object. The resulting images are combined to form a
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